Image handling facilitating computer aided design and manufacture of documents

ABSTRACT

Apparatus for handling digital representations of documents which are to be provided with a number of images. The apparatus comprises a first store for storing digital data defining the image content of each pixel of each image and a color generator for generating data defining, independently of the image contact data, the color of each pixel of each image or combinations of images as defined by a user. A processor selectively combines the image content data and the color data to enable selected images to be viewed separately or in combination as allowed by the layering structure provided.

BACKGROUND OF THE INVENTION

The invention relates to methods and apparatus for handling digitalrepresentations of documents for example to allow sophisticated computeraided design and manufacture of a security document.

Conventional commercial colour proofing relies on the proofing mediahaving yellow, magenta, cyan and black halftone screened colourseparations which match the YMCK inks that will be used for printing.Each ink naturally has to be applied from a printing plate whichcontains the Y, M, C or K information.

The proofs are generally made by exposing Y, M, C and K photosensitivemedia to the corresponding photographic halftone colour separations,using a contact printing method. The individual colour layers are thenmechanically overlaid in register.

Computer generated proofing methods are available for commercial work.One method requires Y, M, C or K halftone photographic separation filmsto be made which are in turn used as masks for contact imaging orproofing media. Another method exposes three or four colour sensitivemedia.

Problems arise with the printing of security documents. Firstly theyinvolve different types of printing processes in the manufacture of anyone document whether, say, a banknote, passport or identity card.Secondly usually one of the printing processes is different from thosegenerally used in the printing industry or is conducted in an unusualmanner e.g. intaglio printing. The printing methods often apply imagesonly with a partial contact of the plate (such as through the use ofschablones) or the inks are delivered in a special manner such as byrainbow printing from split ducts.

It is important to appreciate that most security documents are printedin continuous tone often using continuously blended colours, theblending occuring only on the ink train of the press. Security documentsoften make use of fine line patterns, some delivered in such "rainbow"printing fashion.

In contrast conventional full colour printing (e.g. magazine printing)will take a colour original, colour separate that, convert it into ahalftone format and then print with the three subtractive colours andblack. These subtractive inks are printed at full density or not at all(ie. they are not printed at continuous density levels) and theiroverall appearance depends on the eye blending together interspersedfinely divided areas of the subtractive colours and black to give theperceived colour. This combination thus creates the perception of theoriginal. It is however an illusion, generally unacceptable in securityprinted items.

Although not generally noticeable to the naked eye there has to be asacrifice of the ultimate level of resolution because of the screeningprocess. While coarse colour pictures of bank notes could be attemptedthis way, they would not survive customer's inspection and it isextremely important that the proofed security document should have aresolution similar to the final article--which will be much better thanhalf tone processes allow. It is also important to remember that theinks delivered to the security document are not selected from four orsix pots and only delivered in those colours. Rainbow printing blendsthe colours on the press to give continuous hues as well as continuousdensity.

The methods and materials used in the security printing industry aresuch that genuine documents can be verified such as by visual or machineauthentication methods, and counterfeits or forgeries detected. Thecomputer aided design system employed for security documents must becapable of providing visual representations to an unprecedented degreeof accuracy of colour and resolution so that proofing prints canapproximate more closely to the manufactured counterpart. In particularthe design system must be able to simulate the effect of rainbowprinting. Additionally it is highly desirable that the design system isable to provide visual indications of the placement of special markingssuch as invisible but ultraviolet revealable inks and magnetic inks.

For example a typical banknote will be printed to give a notionalprinting structure of [from the front face downwards]:

    ______________________________________                                                 Serial numbering; monochrome or                                               multicolour/letterpress;                                                      Security design; rainbow, monochrome or                                       multicolour/intaglio;                                                FRONT    Security design; invisible fluorescent/litho or                      FACE     letterpress;                                                                  Security design; rainbow or monochrome/litho;                                 Security design; rainbow or monochrome/litho;                                 Security design; rainbow or monochrome/litho;                        . . .    Security design; rainbow or monochrome/litho;                                 Base paper with optional watermark, threads,                         . . .    marking fibres;                                                               Security design; rainbow or monochrome/litho;                        BACK     Security design; rainbow or monochrome/litho;                        FACE     Security design; rainbow or monochrome/litho;                                 Security design; rainbow or monochrome/litho;                                 Security design; invisible fluorescent/litho or                               letterpress;                                                                  Security design; rainbow, monochrome or                                       multicolour/intaglio.                                                ______________________________________                                    

The printing impressions are placed in the above sequence from the basepaper outwards. There need be no overlap of one impression on another asoften only portions of the note are printed with a given plate.Typically six litho workings will be applied to the document. Finallythe serial number is added by letterpress.

In practice there may be between two and eight lithographic impressions,up to four on each side. Cheques and other security printed items neednot be printed by intaglio but generally will have at least three inkimpressions on one side.

The intaglio impression on each side is generally a single impressionand one plate is used. This plate however is selectively inked by theuse of areas termed "schablones" which need to be cut to size accordingto the colour to be delivered to the design feature. These schablonesmay supply monochrome, multicolour or rainbow coloured ink.

Computer aided design systems which could hitherto be applied tosecurity printing have been limited in usefulness being object or vectorbased. Vector based systems are very useful for producing a component ofa complete document design such as complex line patterns. Vector basedsystems do not readily extend to the introduction of a variety of colourchanges with a given mathematically constructed feature nor can they beuseful for creating images of a complete document.

The LaserScan High Resolution Display device is a vector operated systemin which complex line images are written by a laser onto a photochromicfilm which becomes opaque where the laser strikes it. The resultingimage is then projected onto a screen for viewing; the image is notdisplayed on a CRT screen at that resolution. Such systems are commonlyused in present day security document design and any new design systemhas to be able to import vector patterns from it.

In object based design systems the user specifies the composition of theimage as a series of objects such as lines, circles, polygons which areassigned a position, size and colour. The objects can be manipulatedsingly or in groups and ranked according to whether they are to appearbehind or in front of one another. To display the image the attributesof each object are processed though a programme which computes thecolour to be assigned to each pixel of the display, loading theappropriate numbers into the framestore.

Object based design systems have the disadvantage that they do notprovide the artist with natural working methods; they cannot work withscanned images and the refresh time for complex images is long.

SUMMARY OF THE INVENTION

In accordance with the present invention, apparatus for handling digitalrepresentations of documents which are to be provided with a number ofimages comprises a first store for storing digital data defining theimage content of each pixel of each image; colour data generating meansfor generating data defining the colour of each pixel of each image orcombination of images as defined by a user, independently of the imagecontent data; and processing means for selectively combining the imagecontent data and colour data to enable selected images to be viewedseparately or in combination.

We have devised a new apparatus for handling security documents whichenables these documents to be designed easily while also enabling proofsto be obtained or selected parts of the security document images to beotherwise displayed. This is achieved primarily by handling the imagecontent data separately from the colour data. This not only enablesindividual images or layers to be separately displayed but also enablesselected images to be combined for display and in particular enables incertain applications the colour data to be fixed relative to thedocument while permitting an image to be moved relative to the documentso that the colour of the image will vary depending upon its position onthe document.

The apparatus allows the formation of accurate proofs of all the stagesof banknote printing, so that the sequence of design build up can beseen and if necessary used as a control during preproduction or to showto the customer.

The ability to prepare precision proofs so quickly has a majorcommercial benefit. Design origination in the traditional sense is verytime consuming and the new design apparatus allows origination to occurvery much more quickly.

In the security printing industry it is often necessary to manufactureprinted items in which the register between various printings can bemaintained through a printing run more accurately than is possible withcommercial equipment. Thus for example if three lithographic images areto be printed in register on one side of a document, each having adifferent colour (including rainbow colour) precise registration can beobtained by using a specially designed printing machine which allowseach of the three images to be placed on a single offsetting blanket.This blanket then transfers the three images to the substrate. Incontrast most commercial offset lithographic printing equipment wouldhave to use three offset blankets i.e. using three separate printingstations which is less accurate.

It is usual in the security printing industry to employ a press whichprints images on both sides of a document at the same instant byproviding that instead of each opposing blanket having its ownimpression cylinder that the two blankets directly oppose each other.This allows very fine registration tolerances to be maintained forexample to allow the printing of both duplex effects and see-throughfeatures.

The invention enables these special features to be achieved. It mayfurther provide tolerance parameters such that the designer can view thedocument when layers move laterally within their tolerance bands. Theeffect is achieved by causing the specified image or images to bedisplaced relative to the frame of image reference.

The present invention provides a high resolution design system suitablefor security document design which takes into account the specialmethods, materials and effects used in the security printing industry,which allows more accurate colour proofs to be obtained than hitherto.The system may also incorporate limiting parameters resulting from theconstraints imposed by the particular print manufacturing to be used inproduction.

The present invention can employ the use of a modified "paint" systemwith the advantage that the designer uses electronic controls whichsimulate normal drawing and painting tools. It also provides the abilityto work with scanned in images including those from vector systems andprovides subtlety of results.

In paint systems the designer is provided with a range be of drawing"tools" which closely resemble their traditional counterparts and offerworking methods which directly relate to normal creative practice. Forexample the user can use different brush sizes and load the selectedbrush with a colour mixed to the designer's precise requirements. In oneapplication of this invention by moving the stylus across the tabletcolour is loaded into every cell of a colour store along the path of thestylus while the image content information is loaded into the firststore and the result is immediately displayed on the screen. If amistake is made the brush can be loaded with a background colour and theerror painted over. It also allows the possibility of altering or movingone image with respect to others while maintaining colour registration.Present systems do not allow this.

The first store may contain one binary digit defining the image contentof each pixel of each image. Thus, in this case, the image content isdefined in terms of an "on" or "off" signal indicating for examplewhether or not ink is to be printed within that pixel. Alternatively thefirst store may assign one of a series of permitted levels of signalindicating ink density. This is particularly useful for intaglioimpressions. In other applications some layers can have an on/offcapability and the others multi-level.

In yet other applications where there may be more than one colour in anysingle layer eg intaglio or paper there needs to be a further bit/s perpixel to identify which colour of the available colours for that layerto apply to that pixel. Alternatively this may be done by referencingthe coordinates of a given pixel to a colour data array of similardimensions.

Typically, the colour data generating means comprises a look-up tablewhich has an address corresponding to each possible combination ofimages and contains at each address colour data defining the resultantpixel colour.

The look-up table will be loaded dynamically in accordance with thecolours of the images of the document being designed so as to store ateach address the resultant colour, for example directly in terms of red,green and blue (RGB) colour component values, or indirectly as a colournumber, which is achieved when the selected image or combination ofimages is viewed.

As mentioned above, one important application of the apparatus enablesimages to be moved relative to the colour(s) of the layer concerned.Furthermore, the colour across the document can vary independently ofthe image. In effect when considering a printing operation involving aprinting cylinder and an ink delivery cylinder, the image content datarepresents the surface of the printing cylinder and the colour datarepresents the ink variation across the ink delivery cylinder (althoughthe colour date can also represent rainbow effects).

Preferably the colour data generating means is adapted to generatecolour data for at least one image which varies with position in onedirection across the document. For example, the colour data generatingmeans may be adapted to generate colour data for at least one imagewhich varies with position in orthogonal directions across the document

In general, colour data can be stored as follows:

1 In terms of a colour being assigned to a layer such that any imagepresent on that layer will adopt that colour. This is coordinateindependent and could be used for example for a single colourletterpress layer;

2 In terms of a colour being assigned to image content within a layersuch that one coordinate of the pixel is taken and that is compared withthe colour data. While this could be used to cause a layer to have asingle colour by assigning all positions to the same colour this type oflinear colour assignment is very useful for representing rainbow bandingeffects. Thus if colours could be assigned to all x-coordinate values, afirst set of values could be assigned to a first ink colour (being thepure ink colour). Within the band a number of pixels would be assignedto cover the width of the band and colours calculated by taking therelative contributions of the first and second colours weighted by theirposition in the band to provide a range of blended colours. The next setof pixels would be assigned the second ink colour.

Although a linear array could be employed such that each coordinate hada specified colour held in the data store, an alternative permittingdata compression is to provide a subroutine in which if the coordinatevalue fell within certain limits it would be assigned the first orsecond ink colour value, leaving the precalculated blended colours asdata points. As a further alternative the colour for each point in ablend could be calculated each time it was needed but this is likely tobe slow.

3 Such that colours are assigned on an array basis, taking into accountboth positional coordinates. While theoretically every pixel could beassigned its own colour the array will have particular use in enablingschablone inking areas to be defined on an area basis. The schablonecolour assignment may be made by selecting a colour and assigning thatinto the colour array based on geometric criteria (such as everythingwithin a given square being that colour) or by using a painting methodsuch that the array is given colour corresponding to the area which thedesigner has chosen with an electronic "paintbrush". Alternativelyprocedures may be employed to determine colour based on areainstructions.

For example a composite image of say characters making up thedenomination of a bank note may have a diagonal boundary running acrossthe numbers and intersecting at least some with the number on one sideof the diagonal being represented in one colour and another on the otherside. This would be printed using schablones which apply ink to adjacentportions of the number engraving on the intaglio plate, there being asmall degree of overlap on the diagonal line. The example of thecomputer aided design system just designed would simulate this by havingareas corresponding to the schablones assigned to the two colours suchthat any image pixel would take the colour of its position.

In all of the above it will be common to use colour data (includingcolour number data) which corresponds to previously analysed productioninks i.e. where the inks have been colour analysed into R, G and Bvalues, each typically in the range of 0-255. The designer however maychoose to synthesise inks for example by using a colour blending routinein which two colours can be gradually mixed, selecting a particularcolour. Alternatively the designer may alter or create colours bymanipulating the R, G, B values.

The synthesised and calculated colours will often be non-standardcolours and it is often necessary to provide approximating routinesbecause the monitor display or the proofing printer can only adopt areduced number of options.

If necessary there may be provided a reduced number of colour pointsbeing a factor of the image content points and a concordance routineprovided.

Because the density range of intaglio inks is greater than that possiblewith lithographic inks there may be more than one bit assigned to imagecontent, for example three bits. This allows the possibility of someapparent reduction in image density. Thus while bits indicating fulldensity will adopt the full strength colour, the scaling down will meanthe colour will be reduced in intensity according to the image contentweighting. Thus if the image content was a half the R, G and B valuesfor that would each be halved and that colour or the nearest permittedused.

The above methods may be employed for the assignment of colours forindividual images. When combined images are to be viewed then two ormore layers need to be added. If the image content is merely side byside with no overlap then each image will adopt its own colour.

If the images overlap then generally there are two options. The first isthat one layer, usually the upper layer will take absolute precedenceover the other layer on a pixel by pixel basis. This will mean that thecomposite image will take the colour of the upper layer. This precedencewill take effect when intaglio or letterpress inks are uppermost as theyare opaque inks.

The second option is where one transparent colour (e.g. lithographic) isprinted over another. In this case a translucency weighting factorparameter may be defined in the system such that the net colourdisplayed where the two colour overlap is the weighted average of the R,G and B values (or nearest equivalent permitted colour) of theoverlapped images.

In the case of rainbow blends in two or more the lithographic layers thenet colours (or nearest equivalents) need to be calculated takingaccount of weighting factors.

In addition to storing principal colour (e.g. pure ink) values ifnecessary by colour number requiring the presence of a concordance lookup table the system may also hold a store of the derived colours (orpermitted equivalents) so that when composite image displays are createdthe blended colours are immediately available from a data store ratherthan having to be calculated each time a reference is made.

An alternative method of storing the colour information is describedbelow in connection with FIG. 2.

A particular attribute of the system is the ability for a compositeimage to be displayed and for the designer to be able to alter say thecolour and position of one image within the whole display while allowingthe other images to be unaltered. Thus the designer may be able to moveone of the lithographic images against its fixed rainbow backgroundwhile leaving the other images (those in other layers) untouched.Alternatively the designer may leave the image fixed and move theposition of the rainbow band with the system displaying the net effecteither in that layer or in chosen composites.

The system will generally hold a set of RGB data corresponding to thenormal inks used in production. While approximations of the colourespecially blended colours may be employed for screen display the systemhas the ability to store absolute RGB values such that these values areused separately approximated for screen display and conversion to thecolour proofing colours.

While normally visible colours will be accurately represented the systemmust also allow for secondary features to be represented. Thus printedinvisible fluorescent ink images may be represented in the image contentlayer for that image and assigned either the fluorescing colour or arepresentative colour. Only when that image layer was called for displaywould the image be shown.

For materials which have a visible appearance and a secondary effectsuch as a magnetic effect, there may be provided a further data storefor the special effect in which the effect colour printed can representany real colour resulting from the effect or representative colour whichis intended simply to show the location of the hidden feature.

Watermark images may, like intaglio images, be given a multiple levelcapability for example to make part of the watermark darker than thebase paper and part lighter.

It is convenient to design one face of a banknote using one set oflayers and to design the other face using a fresh set of layers,allowing of course for a common layout and paper (including watermarksand threads) and if necessary lateral image reversal to allow accurateregistration of see-through features, etc.

For the design of some faces of a bank note it is appropriate toallocate two bytes per pixel.

The bits may be allocated as follows:

    ______________________________________                                        BIT              Layer                                                        ______________________________________                                        0        Paper       Background image (tint)                                  1        Paper       Watermark/Thread image                                   2        Litho Ink 1 Design element image                                     3        Litho ink 2 Design element image                                     4        Litho ink 3 Design element image                                     5        Litho ink 4 Design element image                                     6)       Intaglio    Design element image                                     7)                                                                            multilevel                                                                    8)                                                                            9        Letterpress Serial number image                                      10       Letterpress Invisible Fluorescent image                              11)      Scratch     Working/import layer                                     12)                                                                           13)                                                                           14       Layout      Document dimensions, feature                                                  positions, outlines etc.                                 15       Unassigned                                                           ______________________________________                                    

The system may be designed to allow the scratch layer to be the one inwhich most image manipulations are conducted. The scratch layer mayimport rastered images from a data library, an image scanner, a desktoppublishing system or another image creation system. The scratch layermay be operated in bilevel mode where image content is either present ornot or it may be operated in multilevel mode corresponding in the aboveexample to the intaglio image levels.

Generally the scratch layer is used for the working of an image i.e.defining the image content and colouring, but that colour and imagecontent information is usually stored at a specified destination layere.g. lithographic layer 1.

Vector data such as that arising from a high resolution vector designdevice may be imported into a scratch layer which may be operated in avector mode. Any changes to the detail of the image will mean that thedesigner will have to use the modified raster form rather than beingable to utilise the vector data itself. In some applications this ofcourse will be acceptable.

It is, however, possible for the designer to carry out mathematicaltransformations to the vector image, which transformations may be alsocreated in the vector output system by passing appropriate commands ordata to the vector output driver microprocessor. Examples oftransformation include translation, rotation, scaling includingdifferential scaling, windowing or masking in which a vector definedclosed shape is created and image content within or outside that area isused; kaleidoscope e.g. in which a circular segment subtending an anglewhich is a whole number factor of 360 degrees is mirrored at a givenangle and this effect is repeated to complete a whole circle; distortionincluding rectangle to quadrilateral for example to provide a simulatedperspective effect and including also complex distortion in which say arectangular grid is changed to a distorted grid by moving individualgrid points and applying a smoothing function, giving a `rubbersheeting` effect; and stepping and repeating including multiple copyingof a set of vectors into rectangular, polar and spiral arrays and intoother user-defined arrangements such as sine wave, with or layer withoutrotation of the basic module.

The image created by the vector pattern will eventually be converted bya subroutine to the raster equivalent at a resolution corresponding tothe number of pixels in the image content store. This raster equivalentwill often be unmodified in shape by the designer who will simply chooseto position and colour the image, so as to be able to revert back to theoriginal vector data if a platemaking image is needed.

The layout layer allows the designer to specify the document dimensions,registration, feature outline positioning, and lines about which colourchanges will occur. This information may be automatically transferred toall of the other layers so that, say, the border of the document mayclearly be defined without borders having to be created in each layer.The borders of the document will fall within the confines of the pixelarray or may coincide with it. There may be an even border of pixelsprovided around the document which are not used for the design. Withinthe confines of the edges of the document there may be also providednotional margins on at least one pair of opposing sides within which themajor design elements are confined.

After setting, these margins may be applied to a set of other layers andif the margins are permitted to move those in the set automaticallyfollow. A screen display which is not necessarily printable may indicateto the designer where the extremities are and whether the limits arebeing exceeded.

The layout layer may hold registration marks which mark the centre ofthe document but when a proof of a complete document is to be made anyimages such as centre marks associated with the layout layer will havebeen used to create the dimensions of the document being displayed.

To view a layer either the data corresponding to that layer can bedisplayed (duly converted to the output format) or it can be displayedfrom the scratch layer. Two or more layers may be displayed to provide acomposite image such as the tinted paper base layer and the serialnumber layer. Alternatively all of the lithographic layers may berepresented together. If it is necessary to work on one of these layersthen the image content and colour data can be transferred to the scratchlayer and when complete further saved.

The composite document may be displayed by combining Bits 0 to 9 in theabove example. The scratch layer would not normally be displayed unlessa layer had been transferred to it to be worked on. The layout layer maybe displayed as it will hold centering marks, ruling lines and the like.

The invisible fluorescent ink marking may be displayed by turning on Bit10 and either displaying that with the other layers or on its own.

Using a reversal routine one or more images may be turned so as torepresent the view from behind and this reversed image data may be savedand then recalled to be assigned to say Bit 15 so that the design of thesecond face of the note can take into account any registration neededfrom face 1 to face 2.

The apparatus may also incorporate procedures which provide generalbackground indicia which extend from the body of the design into themargin areas. Additionally, the background indicia or a continuousdesign element may extend to the border of the document, continuing onto the next document. The apparatus of the invention may allow suchcontinuous design elements to be shown running over the borders andexhibiting the faces of adjacent notes. Additionally the system mayautomatically cause a continuous design which terminates at the upperborder of a document to be automatically continued at the bottom of thenote in "wrap around" manner. The design may be positioned horizontallyor vertically and procedures supplied to cause the other face of thenote to be presented with the continuation of the image.

This facility allows the designer to see what effect cutting of theprinted sheet into individual documents will have and whether the designis maintained visually. While this run-off may be exhibited on thescreen, the designer may simply display and print only that portion ofthe design falling within the edges of the document.

A further software procedure may allow an array of abutting documents tobe exhibited so that a clearer impression of the printed web at variousstages may be given. In order to display an image of a layer, thevirtual image data for that layer containing image content informationreferenced by layer number and coordinate position must be accessed, ifnecessary by sampling. If there is no image content in the sampled pixelthen that information is transferred to the display. The phosphorequivalent RGB display is made at that position on the monitor's screen.

If there is image content then by use of the layer number and thecoordinate position of that pixel, the colour store may be accessed.

If a layer or combination of layers is to be electronically proofed thenthe colour value for that pixel is translated to a combination ofyellow, cyan, magenta and black proofing inks, each deposited within apixel area at within a range of levels ranging from zero (meaning none)to twenty or more.

The correlation may be made by taking the colour for that pixel andusing the colour number obtaining equivalent yellow, magenta, cyan andblack data which is sent to the ink jet printer at the time of proofing.If necessary the specified colour number may be correlated withpermitted output values.

If multiple layers are to be printed then the layers are firstidentified. The system then establishes whether there is image contentpresent. If there is no image content in any of these layers then nocolour value need be determined or a zero value entered.

Taking a simple case of two layers having possible overlap, the imagecontent of a given pixel if not zero may be of the under layer or thetop layer or both. If either the top or the under layer on its own thenthe determination of colour is straightforward.

Where the two colours overlap the system may allow the upper colour totake absolute precedence over the lower one, that is simulating aneffect where say a letterpress ink was printed on top of a lithographicink, or a proportion of the colour of the lower ink may be combined withthe colour of the upper ink to provide a new colour value. Thissimulates the effect of printing one transparent e.g. lithographic inkover another. The weighted calculation may be undertaken by taking theRGB values of the first colour and combining with a proportion of theRGB values of the second colour.

This derived colour value may then be translated into a permissiblecolour number for the system to use. Similarly for proofing theindividual yellow, magenta, cyan and black values may be calculated.

Thus the colour of a given image pixel may be determined by using simplythe layer number if all of the images within that layer may have onlyone colour. Alternatively the colour of a layer may be constant but thecolour data may be held as a linear array having a dimensioncorresponding to the number of pixel points available on that axis. Eachof the points of the linear array may be assigned the same colour.Alternatively a two dimensional array of points corresponding to thenumber of image pixels may be provided, each of the colours in the arraybeing set to the predetermined value.

A routine is then provided such that the colour of a pixel is determinedby reference to x, or y, or x and y coordinates.

The designer may change the colour for each point, be linear array ortwo dimensional array by choosing another colour. Generally the designerwill be provided with a series of preselected colours, defined as colournumbers which are correlatable to a table of `absolute` red, green andblue values obtained by colour analysis correlatable with the printinginks to be used. Derived principal colours may be calculated bycombining the respective RGB values of, say, two principal colours incontrolled manner to provide the derived RGB values which in turn may beassigned a colour number. To maintain a reference of prime colourvalues, it is desirable to store the derived RGB values in that form soas not to lose colour information from the storage data. By the abovemeans a series of colours can be defined in RGB terms and assigned.

Similarly when the proofing is to occur the colour number from thecolour data stack or the absolute levels of RGB components need to beconverted by a subroutine to equivalent subtractive colour values.

Again approximation may need to be employed because of the limitednumber of colours printable using the permissible combinations of inksfrom the proofing printers.

In addition to the principal colours and derived colours, the system mayalso hold alternative colour data for any layer or part of layer such asto indicate the effect of special materials. For example an invisiblebut ultra violet revealable dye would have no colour assigned in theprimary colour store but, if it fluoresced blue, a blue colour in thesecond (alternative) colour data store. A photochromic ink would haveits normal colour assigned in the primary colour store and itsphotochromically activated colour in the second store. A magnetic inkwould have its true colour shown in the first colour data store andsimply an identifying colour in the second data store which wouldindicate where magnetism was present.

The first and second colour data stores will have layer and generallypositional referencing and they may be combined. Alternatively, theremay be two separate data sets. In order to select either the first orthe second colour the image content store may additionally include apointer as to which colour to employ.

While the system has the inherent capability of working with a widerange of R, G, B values including where the designer may wish to definecolours by choosing R, G and B values, in practice the presentlimitations of RGB phosphor displays and yellow, magenta, cyan and blackproofing inks mean that approximations have to be employed. Theapproximation may be made by reducing the number of colour numberoptions in the colour data store or providing display or proofing.

In practice, the colour monitor may display two hundred and fifty six(256) colours and the colours in the data stack need to be reduced tothat number by approximating, i.e. ascertaining whether colours areprecisely those permitted or whether they simply fall within a groupwithin which all members are assigned a unique number. This uniquenumber may be used as a reference to change the colour in the data stackto the approximated colour if necessary or an approximated colour datastore may be created.

In general, any particular display device will be unable to display allpossible colours which can be printed. Preferably therefore theprocessing means includes means for adapting the combined image contentand colour data for use with a display device.

For example, the adaption means may comprise a look-up table for storingcontrol values suitable for use with the display device, the look-uptable having an address corresponding to every possible combined imagecontent and colour data value.

In those cases where the images are printed, the digital data mayrepresent printing ink density. In addition image data sets representingbase paper (including a watermarks, security threads etc) will also bestored. The image data set representing the paper may use more than onebit per pixel to allow normal, darker, lighter shades and securitythreads to be represented. The term "image" should be taken to mean theindividual components which are to be inked etc. Thus in the case of thespecial offset lithographic effect where three images say are deliveredonto a common blanket each of the components would be regarded as animage (having its own layer). The composite image delivered from thecommon blanket would not be regarded as an individual image although itis applied during one print cycle. The term "image" should also be takento cover any basepaper tint, watermark or thread etc. in the paper andthe serial number (and indeed any label of hot stamped foil which isattached.)

The representation of each layer digitally and in pixel form thusseparately allows a number of significant applications and processes tobe achieved in design and manufacture which provide a significantadvance on the art.

The apparatus allows the designer rapidly and with high quality tocreate designs by using computer simulations of the construction methodstraditional in banknote design and to be able to view these on thescreen and modify them if necessary. Individual layers or images can bemanipulated without overwriting other layers. Colour proofing prints maybe generated at various stages. Individual layers (or their rasterorigin or vector origin components) can be viewed singly or incombination with other layers. The system is interactive allowingindividual layers to be manipulated at all times. For example, imagecontent data can be copied or moved from one layer to another.

The system has been designed not simply with "design" only in mind butto allow platemaking and to incorporate production constraints inherentin specific types of printing presses which will be used for printingthe documents. For example parameters may be defined which warn thedesigner if serial numbers are being placed in position which would falloutside a given printing press's tolerances. These parameters are storedas data by the apparatus. It can also incorporate design constraints.For example parameters may be defined which warn the designer ifnon-standard paper sizes or colours are being employed. These parameterswill also be stored as data by the apparatus. Furthermore, the apparatusmay allow a degree of positional tolerance of one image relative to theother. For example while four litho workings on each side of thedocument can be placed in very precise register, as they can from backto front, the registration from, say, litho to intaglio or watermark tolitho, will be less precise.

The apparatus may allow the tolerance to be viewed by determining fromthe production tolerance data what the positional limits are andpresenting a set of images at either extreme or without any offset.

This is useful because the designer can then see if a particular effectwill in fact blend when its relative position varies. Similarly theprecise positioning of the watermark and thread will vary from note tonote, but within known limits. By reference to a set of predefined datathe apparatus may allow a warning to be given if the designer tries toexceed the production limits.

By using the image content data alone, the system also allows output ofthe image data alone, without colour, for example to allow black andwhite negatives of the layers to be prepared for platemaking purposes.Alternatively the output device e.g. a laser or high precision plotter,might be used to image or engrave plates directly. In this case thevector data employed in the high resolution plotter may be retainedready for production use with the pixel equivalent image used for imagemanipulations. The complete apparatus need not be present on one site.Completed design data can be sent to other sites by data communicationmeans, if necessary protecting the data using encryption, to allowproofing at an overseas location.

Other capabilities which can be represented electronically include

a) image reversal capability; this is to allow face to face, i.e. frontto back registration, of major importance in the design of most securitydocuments. The image reversal capability may be operated by firstlychoosing an image data set which will be in pixel form, secondlydefining the axis of transposition for that image within the positionalreference frame and then operating a routine which sequentiallytransposes the image content data of given pixels to the transpositionpositions thereby creating a new data set of the transposed image, whichmay be stored and further electronically manipulated. In most cases thetransposition will be undertaken about a vertical line running midwaydown the middle of the area of the document. The colour will changeaccording to the new position of the pixels.

The image reversal capability may be used to cause images to face in theopposite way, to provide mirror images and to provide a registrationguide for complementary images such as see-through or print throughimages which are printed in close register on both faces of a note.Where two images are required to be in register from one layer toanother, they may be so identified so that a subroutine can move thecomplementary image automatically to maintain register when the firstpart is moved.

b) Orlof capability: the system allows the designer to create an imagewhich suddenly changes colour. For example a line may cross the note. Inthe middle it will suddenly change from one colour to another. Toachieve this the line portions have to appear in precise register in twodifferent colour layers. This is an important and unique feature. TheOrlof capability may be operated by firstly creating the complete imagewithin one layer, defining a colour splitting line which cuts the imageand electronically transferring that part of the image corresponding tothe second side of the splitting line to a second layer so that may becoloured differently while retaining the first part of the image at thefirst side of the splitting line in the first layer, and colouring it.The colours chosen for the first and second image portions will bedifferent. By providing the splitting of the first and second imagesagainst a fixed frame of positional reference, close register of thefirst and second images in their respective first and second layers maybe achieved. A simple procedure may also be employed to allow thesplitting line to be moved in either the first or the second image suchthat the image data is automatically transferred from the first portionto the second portion or vice versa. The designer may thereby experimentwith the effect until the optimum splitting is obtained. The wholedesign area may also be translated in position from one part of adocument to another by using a subroutine which maintains registration.

c) Fade capability: the system allows image content to change from onecolour to another. Thus a word printed in the horizontal may have adiagonal colour wave, say, allowing the top right hand corner to beturquoise and the bottom left to be green with a gradual transitionbetween the two (rather than a sudden Orlof type change). The fadecapability is somewhat similar to the Orlof capability in that two imageportions abut but in this instance, instead of a specific splitting linebeing employed, the apparatus allows a splitting band to be defined suchthat the adjacent colours merge within the band in a visual manner akinto the effect of rainbow printing.

The orientation and width parameters of the band may be determined bythe designer.

d) The system can also have or import data from a by microletteringcapability by using the image content data facility. This will allowlines to be created which on and magnification will be seen to containalphanumeric characters, symbols, logos and the like, for example, thename of the issuing Bank continuously repeated in fine lettering. Themicrolettering capability is operated by creating a character set, suchas alpha numeric characters, symbols, lines and other indicia, andpresenting that in positive or negative usually in a line format. Arepeat frequency of the image is usually declared such as if the name ofa bank is to be indefinitely repeated along the line. This may beimplemented by using a step and repeat procedure to create the line,which may then be positioned with a reduction scale factor applied sothat the effect is of a line. In the proof or on the screen the line maybe represented apparently as a normal inked line with the microletteringtext presented in negative against a background or as a positive. Theline may be employed as a framing line. While if very small themicrolettering may be unresolved on the colour monitor or the proofingprinter, the design apparatus may allow portions of the document to beexhibited magnified. In such instances, the data set corresponding tothe microlettering may have an apparent resolution greater than isprovided by the resolution available over the rest of the document.

Microlettering may be provided on security thread simulations either byproviding positive or negative images. It is also possible to position anumber of lines together using a step and repeat routine so as to coveran area. This filling type of routine may also employ the facility ofcausing the set of lines to follow a curve or other geometric design.

e) Through the use of layering within the image content data the systemcan have a duplex/triplex capability; this will allow creation of acomplex design component extending in precise register of line andcolour over a few layers. Duplex and triplex elements occur where thereis a geometric pattern such as a grid made of lines, into the cells ofwhich are separately printed in precise registration colour patcheswhich have the shape and size of the cells or are slightly smaller. Onepart of the design is common to both portions such that when printedthat the second printing of the one part overlaps the first, resultingin a third colour. This is very difficult to reproduce commerciallyusing conventional techniques and there may be see-through orprint-through associated printing on the reverse of the note. Any designmay be imported into various banknotes as a common element, either froma pixel image store or from the pixel equivalent of a vector design. Theduplex/triplex procedure allows complementary portions of a given designfeature to be allocated between two layers, each of which is held inprecise and complementary register such that both parts may be givendifferent colours. Preferably a procedure is employed to cause themovement of the other part of the image to maintain register if thefirst part is moved.

The splitting of a composite triple image into its individual componentsallows a triplex image to be simulated.

f) See-through printing occurs where two faces of, say, a bank note areprinted with precisely complementary images such that on viewing thenote from one face the printing on the opposite face of the note doesnot significantly appear. Print-through printing occurs where theprinting on a part of one face of a note is complemented by similar butnon-identical printing on the other face of the note such that onviewing from one face of the note the printing on the other face can beseen accurately to be in register.

The apparatus of the invention allows this effect to be simulated byproviding an image reversal routine as described previously which thenmay be used in colour muted form to provide the basis for viewing thefeature from the other face of the note. The analogy with duplexprinting will be apparent. A positional translation procedure may beemployed such that when the designer moves one part of a see-throughfeature or a print-through feature, the coordinates of the complementaryimage data of the other layer area are automatically changed.

g) A further software routine may be provided to take a specific outlineand then provide a series of margins of different colour around thatshape.

Thus, the system allows the outline to be defined and then after savingof the outline, a margin is radially defined with a specified width.

This margin may be saved as an image and given a specific colour.Further margins may also be provided if necessary, the width of themargin reflecting adjacent design features by providing a routine whichassesses the distance from the perimeter to the edge of the note or toan intervening design feature on a radial basis and using that todetermine the size of the margin. Multiple margins may be formed toprovide a series of `growing outlines`.

h) Medallion effects may also be represented. Medallion effects arecreated by the designer by causing a stylus to track over a threedimensional surface. The upward movement of the stylus causes theconnected drawing stylus to be deflected within its drawing plane by anamount which corresponds to the vertical deflection of the first stylus.Within the apparatus of the invention this effect may be represented byusing a segment of a line corresponding to a notional upward deflectionto be slightly displaced in parallel to the line by an amountproportionate to the extent of notional upward deflection.

i) An image interrelation feature can be achieved to allow a definedarea to be specified which covers identical or complementary imagesfound in two or more layers such as are described with see-througheffects. By linking the two images it is possible to secure thefollowing of the second image to the translational movement of the firstimage. This is achieved by providing a routine which changes theposition of the second image within its image store. If necessary, thismay be done by defining only a portion of a given layer and moving thatrather than securing movement of the whole layer.

Another problem with handling graphics for security documents arisesfrom the manner in which security documents are printed. Thus,typically, a security document is printed in a series of separate printoperations some of which involve the laying down of a printing inkwhich, in many cases, will overlay all or part of a previously printedink. In the past, when a computer generated proof has been produced of asecurity document image it has been assumed that the most recentlyprinted ink completely obscured all inks printed beneath it and thus agating system was used in which only the colour of the topmost ink wasprinted on the proof. Such a system is described in GB-A-2180427 andEP-A-144138. With the present invention the contribution of underlyinglayers to the resultant colour is recognized.

Examples of different types of operations which can a produce an imageon the finally manufactured document include:

intaglio printing,

lithographic printing including wet and dry offset

methods,

letterpress printing including serial number printing,

screen printing,

gravure printing,

flexographic printing

laser induced colour transfer printing,

thermal element induced colour transfer printing,

laser induced colour generation imaging (positive or negative),

thermal element induced colour generation imaging (positive ornegative),

electronically applied colour pen imaging,

electronically applied ink jet printing,

electronically applied bubble printing,

electronically applied ribbon printing,

xerographic imaging,

embossing, and label affixing (including hot stamping of transfer filmsand foils).

Other factors which may be simulated include watermarking, fibre,planchette and thread provision ie. the image is provided duringcreation of a substrate. The images could also be normally invisible inthe final security document, as described below, but selectivelyrevealable.

The invention is particularly suited to the production of proofs byelectronic printing. Preferably, the processing means is adapted togenerate from the image content and colour data proofing control signalsdefining the corresponding image or combined images in terms of proofinginks or dyes. Typically, the proofing inks comprise two or more(preferably all) of cyan, magenta, yellow, and black.

Preferably, the deposited proofing ink densities are defined on amultiple level scale having at least three values, more preferably atleast ten. Further, the electronic printing operation (for generating aproof or final product) preferably comprises depositing a quantity ofink in each area corresponding to a pixel, the quantity of ink depositedcorresponding to the data value for the pixel.

In conventional digital colour proofing, each output pixel is eitherprinted with a full, single density ink value or not printed, therebysimulating halftone printing. This halftone proofing process involvessignificant loss of image content information and is less likely towithstand inspection under magnification. We have appreciated, however,that with the complex images which are devised for security documents,it is particularly necessary to be able to generate a proof with asimilar colour resolution and colour accuracy to the finally printeddocument. We have appreciated that this combination of better resolutionand better colour accuracy can be achieved by printing the proofing inksat a variety of different densities. Typically, there will be at leastfive different density values, more preferably twenty values, and mostpreferably at least thirty values. However, even larger values such as256 could be used. Different densities may be achieved by applying inkto different areas.

This extends the available colour range to several thousand differentcolours and offers almost continuous tone capability. In one preferredarrangement, an ink jet printer is used to generate the proof since thiscan deposit accurately controlled quantities of the proofing inks torepresent different density levels. The difference in density isachieved by varying the thickness and/or size of the deposit accordingto the desired hue.

In the preferred arrangement, the apparatus further comprises aframestore for connection to a monitor, the processing means beingadapted to store the combined image content and colour data in theframestore in a manner suitable for controlling the monitor.

Typically, the data stored in the framestore will define monitor colourcomponents such as red, green, and blue. These will usually be differentfrom the printing inks when a full document is displayed on a highresolution colour monitor the exhibited resolution will generally beless than that inherent in the primary data store of image data. Thisprimary data store is used for proofing out put, rather than the derivedframestore data.

Security documents generally possess a series of different measures forauthentication and against counterfeiting. Some of these will beimmediately viewable and will be primary proofing images, and others,secondary images, will be revealable to the eye such as fluorescent inksrevealed by the use of ultraviolet light, photochromic inks temporarilyrevealed by high energy light exposure, thermochromic inks revealed byheating, metameric inks revealed by the use of a suitable viewing filtersuch as an infrared filter for a pair of red metameric inks, andchemically responsive inks and coatings. Other secondary images will bedetectable only by machine sensing such as magnetic inks whether theiroverall deposition or codable areas, X-ray opaque inks andspectroscopically revealable inks. Penetrating inks in which there aretwo components one which colours the surface, and the other whichcolours the underlying area and perhaps border of the overlyingcomponent may also be used particularly for serial numbering.

Metallic inks and the above revealable effects will not be reproducibledirectly by the proofing method which will generally employ onlyconventional coloured inks. Such secondary effects may however be shownon a complementary series of proofs where the secondary effect areas areidentified in distinguishing colours to show their position and suchcolour effects as the inks provide.

Many documents will contain metallic ink printing to provide a defenceagainst copying. Normally metallic areas in a proof will be shown indistinguishing colours but in certain instances it may be possible toobtain a proof by imaging a metallised substrate such as a substratepreviously printed with metallic ink in the appropriate area.

The system can thus provide proof of prints which provide a visualindication of the location of concealed features such as the invisiblefluorescent inks mentioned above. These concealed features may beprinted in any suitable identification colour and may simply be printedon an otherwise blank document shape or on a version of the otherwisecomplete document except that any visible colour associated with themarking will be replaced by the colour identifying the presence of aconcealed feature. In the case of photochromic inks which can beassigned two visible colours then these two colours can be used.

Thus an advantage of the system is that it can not only reproducespecial printing colours but also give a visual indication of thepresence of special materials.

Proofing may occur on white paper, or tinted paper or indeed on paperwhich carries a real marking such as a watermark or thread or metallicfoil (optionally embossed or embossable) label and all these can besimulated by suitable image content and colour data.

A feature which is sometimes incorporated into a security document toimpede copying is a rainbow printed design. Rainbow printing is a termused widely in the security printing industry. What is meant is that twoinks are provided in a common ink duct separated by a divider. As thepress turns the two inks are delivered into their respective parts ofthe inking train. As a result of the lateral oscillation of the inkingrollers in the train the two inks are blended in the inking train toform a continuous gradation of colour across the blending band. The inkthus mixed on the inking train is offered to the ink receptive surfaceof say a lithographic printing plate with the result that an imagestretching from one pure ink colour to the other across the band becomesinked in a banded manner and this banding is transferred to thesubstrate via in offset lithography a blanket. This effect is difficultfor counterfeiters to produce.

In any press there may be more than one pair of inks blended so thatacross a given document there may be two or more bands.

In one highly preferred example of the invention, rainbow images can berepresented by storing in a second store digital data defining one ormore colour bands for some of the layers in which there is a gradationin colour, wherein the data defines respective ones of the printing inksin pure colour form in unmixed regions of the band and representsgradient combinations (eg. linear or logarithmic) of the inks betweenthe unmixed regions.

In this new system a single layer may be arranged to exhibit rainbowprinting effects by creating two or more separated colour bands ofdifferent colours and allowing the colours to merge in gradient fashionbetween one band and the next. The position and width of each colourband is determined by the operator and the band colours are selectedfrom the available range of predetermined, ink matched colours. Colourin the band between the two predetermined colours (or where they overlapif abutting) is then calculated by means of a mathematical gradientexpression.

In simplest terms the colour contribution of the expected printed colourdensity from the first of two abutting inks in the ink duct varies onthe printed document from its unblended (maximum) value at the firstedge of the first blending band and this linearly decreases to nothingat the far edge of the blending band. Similarly the colour contributionof the second colour is allowed to decrease linearly in the oppositedirection, if necessary taking account of the different ink spreadingcharacteristics. By combining these colour contributions and taking intoaccount weighting of the colours on underlying layers, the final colouris calculated per pixel and stored. RGB values are used for display andfor correlation with the ink jet printer YMCK parameters.

The gradient of colour change in the blending area may be of a linear orother simple mathematical nature or made in a series of steps.

Reference data or mathematical expressions may also be provided to allowcolour calculation where one rainbow impression is placed over another.For example there may be masking factors assigned to each ink dependingon its opacity as previously described.

Rainbow bands generally run parallel to one edge of the final documentand will appear as horizontal or vertical bands.

The processing means may be adapted to generate from those data setsrepresenting printing operations, control data for controlling themanufacture of a printing plate.

The stored image data may be used for platemaking

i) by writing directly onto a plate for each layer e.g. by laser;

ii) by exposing a photographic film which is then used as a mask forplatemaking e.g. by laser;

iii) by outputting data to a subsidiary computer controlled platemakingsystem e.g. by laser; or

iv) by outputting to a vector plotter or pixel plotter.

DESCRIPTION OF THE DRAWINGS

Some examples of methods and apparatus in accordance with the presentinvention will now be described with reference to the accompanyingdrawings, in which:

FIG. 1 is a block diagram of one example of the apparatus;

FIG. 2 illustrates schematically the arrangement of the image store andcolour look-up tables shown in FIG. 1;

FIG. 3 illustrates a monitor and keyboard; and

FIG. 4 illustrates an example of a monitor display.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The apparatus shown in FIG. 1 comprises a processor 1 which iscontrolled by a user via one or more input devices 2 such as a keyboardor digitiser. The processor 1 accesses image content information in animage store 3 and colour data in look-up tables 4. Images can be viewedon a monitor 5 coupled with the processor 1 and proofs can be printed byan ink jet printer 6 coupled with the processor 1. A further store 7 isprovided for storing a library of previously created complete or partialimages, the store being connected to the processor 1 via a networkadaptor 8. In addition, the processor 1 may be connected to a vectorpattern generator 9 such as a laser source, to make film masks forplatemaking in a vector output device 10.

Additional devices may be connected via the adaptor 8 with the processor1 such as raster image input devices selected from a colour rasterscanner, a video camera and the like which enable a raster colourseparation of an original image into red, green and blue components; orvector input devices for example selected from a pressure sensitivestylus operated digitising tablet, a tablet and puck, a mouse, or avector input from a separate computer system. The apparatus isparticularly suited to the proofing of security document printing imagessuch as banknotes.

The designer will use the video display unit monitor 5 to displayaspects of the design. The screen will give a colour representation inred/green/blue phosphor the emissions, which is matched to the originalcolours and that of the final inks. The screen-to-final-ink matching isless critical in colour accuracy and resolution than would be neededfrom proof-to-final-ink.

The designer is able to view individual print impression layers orimages and composites including the completed note.

The designer is able to incorporate both raster information and vectorinformation. Vector information such as that arising from the use ofhigh resolution pattern generating equipment which may output by drivinga laser beam to record on photochromic film geometric line patterns, isless consuming of computer memory but requires conversion to raster(i.e. pixel) information.

The system is provided with a "scratch" layer in which to create graphiclayouts from either vector or raster inputs, which then can betransferred to a specific ink impression layer or other feature layers.

The scratch layer will permit origination unconstrained by print relatedfeatures. Thus the scanned raster data and mathematically constructedvector data may be loaded from a network and manipulated vector databeing subsequently converted to raster and merged into another layer.FIG. 2 indicates schematically the image content store 3. This store 3(the Virtual Image Store) contains a location for each pixel of theresultant design at the highest resolution required (the Virtual Image).Each pixel is defined in terms of a sixteen bit (two byte) data valuewith each image (or layer) which is to be displayed or subsequentlyprinted represented by one or more bits. This is shown in FIG. 2 where

    ______________________________________                                        Bit No.      Image Type                                                       ______________________________________                                        0,1          Paper including Watermark, etc                                   2            Litho 1                                                          3            Litho 2                                                          4            Litho 3                                                          5            Litho 4                                                          6,7,8        Intaglio (DP)                                                    9            Letterpress                                                      10           Invisible Print                                                  11,12,13     Scratch                                                          14           Layout                                                           15           (unassigned)                                                     ______________________________________                                    

One of the advantages of the invention is that any one single layer canbe viewed or proofed alone or a combination of layers can be viewed orproofed. Accurate colour rendering is achieved by the use of proceduresthat examine the layers currently selected for viewing or proofing andautomatically set the appropriate colours in the display framestore orprint file.

Display resolution is generally lower than that of the Virtual Image, soit is only possible to display a subset of the Virtual Image pixels onthe monitor 5. When viewing the whole image on the display this isachieved by sampling the Virtual Image pixels at an appropriate ratio.For example, if the Virtual Image contains approximately 4000 ×3000pixels and the monitor resolution is 1024×768, then every fourth pixelis displayed from every fourth row.

Typically, up to 256 different colours can be displayed by a monitor atany one time. In the case of the display of a single image layer, areasonably accurate display can be achieved. However, when more than onelayer is combined it may not be possible to display all of the resultantcolours so accurately and a procedure has been devised to make best useof the available colours. This procedure for assigning display colourscomprises two stages: Setting up the colour tables, and updating thedisplay. FIG. 2 illustrates in more detail one arrangement of the colourtables 4 shown in FIG. 1. The following data structures are used:

a. The Display Colour Table--DCT (14). This typically has 256 entries,of which about 40 are used for the fixed ink colours chosen by thedesigner, about 200 are allocated dynamically to the colours produced byblending in rainbows and transparent layer overprinting, and theremaining 16 are reserved for displaying menus, cursors etc.

b. The Intermediate Look-Up Table--ILUT (11). This has 32,768 entries,indexed by each of the possible combinations of the 16 bits thatrepresent each pixel. The contents of each location in the ILUT aredynamically allocated to indicate which entry in the Display ColourTable 14 should be used to represent the appearance of a pixel whose bitpattern corresponds to that location.

c. The Litho Colour Data Table--LCDT (13). This comprises a set ofone-dimensional arrays, each having as many entries as the number ofhorizontal pixel addresses in the Virtual Image, typically 4096. The setcontains one array for each possible combination of litho layers inwhich colour blending and overprinting are to be represented. Forexample where there are four such litho layers, sixteen arrays areprovided in the Litho Colour Data Table.

d. The Colour Pointer Table--CPT (12). This has the same number ofentries as the Display Colour Table. Entries are either set to zero,meaning that the corresponding entry in the DCT(14) is valid, or containa pointer to one of the Litho Colour Data Tables.

These colour tables are initially loaded with default values, and areupdated each time the operator changes any of the ink colours, orchanges the position or width of any of the colour blending regions, orchanges the combination of layers to be displayed. The updatingprocedure is as follows:

First, the Display Colour Table 14 is loaded with predefined RGB datafor the fixed colours of inks and paper that the designer has selected.These will normally occupy the first 40 or so locations in the table.

Next, the number of different litho colour blends and overprints iscounted for the currently defined rainbow band splits and the currentlydisplayed layers. The 200 available entries in the Display Colour Table14 are then allocated to these combinations and the actual colours arecomputed according to appropriate conventional blending and opacityalgorithms.

The contents of the Litho Colour Data Table arrays 13 are then set sothat each location contains the index M of the entry in the DisplayColour Table 14 that corresponds to the colour for that horizontalposition and combination of litho layers.

The Intermediate Look-Up Table 11 is then set up. For each location inturn, the layer combination that it represents is examined to determinewhether the topmost layer of those currently selected for display is an"opaque" layer (i.e. Layout, Scratch, Invisible, letterpress, DP orPaper). If it is, then that entry is set to the DCT value (in the range1 to 40) corresponding to that layer's ink colour. If on the other handthe topmost displayed layer is a litho layer, then its ILUT entry is setto the DCT value (in the range 41 to 240) corresponding to the leftmostcolour for the currently displayed litho layer combination. (This is aprovisional setting only - the actual colour to be used will later bedetermined by reference to the LCDT).

Having set up the colour tables, the display is updated. For each pixelthat is to be displayed, its 16-bit value is used to index theIntermediate Look-Up Table. The value N (which will be in the range 1 to256) found at that ILUT index is checked against the contents of the Nthlocation in the Colour Pointer Table. If the CPT entry P is set to zero,then the pixel is assigned the colour contained in the Nth location ofthe Display Colour Table 14. If the CPT entry P is non-zero, then thex-coordinate of the current pixel address is applied to reference theappropriate array P in the Litho Colour Data Table 13 for the currentlydisplayed combination of litho layers and the contents M used to indexthe Display Colour Table 14 for that pixel.

As explained above, the data stored in the virtual image store 3 canoriginate from a vector pattern generator 9. If the Scratch layer isselected for display and vector mode is active then any such vector datais converted in a conventional manner to bit map form and then drawnonto the screen In addition, the vector data may be stored in the store7 and can be used directly to produce printing plates as indicated inFIG. 1 by the output device 10.

The procedure for creating a print file for proofing is much simpler.The resolution of the printer will generally be the same as that of theVirtual Image so that every pixel will be printed. Also the printer willgenerally accept colour signals with the same data resolution as thatwhich is used to define the ink colours in the ink colour database (notshown), i.e. one 8-bit byte for each of three components. Thesecomponents will generally be RGB, or YMC with optionally K, but anyother triplet system of colour specification (for example Hue,Saturation and Lightness) may be used with appropriate mapping tables.

To define the colour of each pixel in the print file, its 16-bit value15 (in FIG. 2) is masked to remove the contribution of those layers notcurrently selected for printing. If the topmost printed layer is a litholayer, then the colour of that pixel is computed according to itsx-coordinate, taking account of the position and extent of any blendsand using the litho ink colour data and opacity factors for the activelayers. Otherwise, the colour of the topmost layer to be printed isdeemed to be opaque and its colour definition is used alone.

The scratch layer may have a masking-by-colour feature. Thus areas canbe "masked" according to their colour so that they cannot be overprintedby colours with a lower mask number.

Although the system may present all of the front and back layers to thedesigner at one time it has been found useful to work on one side of thenote, typically with six layers, and then save that data before workingon the six layers of the other side, taking into account front-to-backregistration by, say, the reverse of one layer from the opposite face.

The apparatus thus allows the design of a variety of document faceswhich are comprised of one, two, three, four five, six, or more imagesdepending on the requirements of the document under design.

Thus for the above structure the electronic layer available to thedesigner for one side may be:

1. Layout layer (for defining the dimensions and registration of theitem)

2. Scratch layer

3. Serial numbering; monochrome or ie multicolour/letterpress;

4. Security design; invisible fluorescent/litho; letterpress

5. Security design; rainbow, monochrome or multicolour/intaglio;

6. Security design; rainbow or monochrome/litho,

7. Security design; rainbow or monochrome/litho;

8. Security design; rainbow or monochrome/litho;

9. Security design; rainbow or monochrome/litho;

10. Base paper (including watermark and thread)

Each of the above will be independently viewable and proofable as willspecified combinations. Vector images are defined in a separate layer(e.g. the scratch layer) from which vector objects are then "painted"into the desired layer in raster mode only.

There may also be invisible or other hidden feature layers.

Thus the image of one layer on one face of the note may be transferredin left/right reverse mode (i.e. the rear view), temporarily to one ofthe layers of the second face of the document to allow accurate relativepositioning. In general it is only required to check see-throughfeatures and intaglio placement and it is adequate to undertake thiswith one layer at a time. It is however conceivable to use the compositeof the whole side or a selection of layers.

The system will allow each face of the note to be viewed and proofed infull colour. When the layers are viewed in combined format means areprovided to ensure registration of the front layers with the back.

Colour fidelity is important in colour proofing and the processor 1 isable to improve upon the known gating method in which one overlyingcolour takes absolute precedence over another. In the new system, aseries of comparative data are provided which allows the overall effectof overprinting one colour with another to be accommodated.

This data is prepared by providing via colour microdensitometry a seriesof data relative to the densities of each printing ink which is to beused and then providing a further series of data from inks at variousdensities and in various orders of printing. This is then related to theproofing printer ink densities and optionally the video display unitappearance. When pixel colour parameters are determined in the design,the combination of those layers and colours is then matched against thedata to allow the empirical colour then to be provided to moreaccurately represent the true tones.

The colour balance may be achieved by use of reference data tables oralternatively from mathematical combinations, or by a combination of thetwo techniques.

Thus in any given impression layer rainbow printing may be achieved bycolour calculation (or reference data). In any composite image of say afinal banknote design the contributions of colour from underlying layersmay be taken into account by reference to data tables or mathematicalcombinations.

The use of colour correction data enables accurate proofing colourreproduction of the designer's colour crayon drawings, the lithographic,letterpress and intaglio inks, the paper colours and security featurescontained therein.

To do this the colour crayons, the YMCK proofing inks at variousdeposition settings and the printing press inks at various thicknessesare measured by colour separation microdensitometry which then gives acommon comparative basis for correlation. Such measurements also takeaccount of one colour deposited on top of another. If a composite colourvalue is determined for a pixel by the computer a search is made to seeif there is a corresponding, say ink jet value which allowsdetermination of the actual ink jet settings to be used for proofingthat pixel. If there is no precise value, an approximation may be madeautomatically from neighbouring values, although care has to be takenwhere the neighbouring values are white.

As an alternative, colours may be given masking values. Thus this allowsYMCK to be overprinted only if the top layer YMCK colour has a highermasking value.

The input information may be in raster or vector format. The finaloutput must be in raster format for the colour proofing equipmentalthough any vector information can be retained for subsequent use ormodification as the design develops.

The colour proof may be prepared by using combinations of four colours(that is yellow, magenta, cyan and if necessary black) which aredeposited in precise registration on the proofing paper by a four colourink jet printer which operates in a matrix fashion (as distinct fromcontinuous tone or halftone). An example of such a printer is the Irisink jet printer made by Iris Graphics Inc., USA as incorporated in theCrosfield Electronics Limited jetproof proofing system. The compositecolour data allows multilevel control of the quantities of the jettedinks to be deposited, for example by providing 31 steps of drop size foreach ink colour.

The system may also be coupled to image recognition equipment, such as aCCD array detector arranged to capture either a security printing plate,or security printed (completely or partially) substrates. By use ofsuitable comparative circuitry the captured images can be compared withthe designed image to allow automatic document inspection at any stageof the process.

The colour proofing images which may be obtained from the apparatus viaa colour printer, such as an ink jet printer, may be presented indifferent ways.

In most circumstances printed metallic inks will have to be depicted ina suitably distinctive colour as metallic proofing inks are not readilyavailable.

Although somewhat difficult to prepare in a proof, the system has theinherent capability of handling materials which have degrees of opticalvariability. The ability to illustrate the position of simulations ofsuch features in proofs which have been printed on special paper ispotentially useful because of the use of special features in securitydocuments.

Examples of optically variable features include

metallic or coloured metallic printed portions;

metallised portions (e.g. threads or metallic labels such as hot stampedlabels);

coarsely embossed metallic print or metallised portions which Wayinclude the deposition of ink in the embossed pattern, or may beembossed without ink (such as latent and transient images whether singleor multiple, or these images accompanied by a touring line) embossedmetallised portions (including holographic effects), diffractivegraphical patterns, diffractive geometric patterns and matrices ofthese);

optically variable ink portions comprising thin film opticalinterference coatings;

photochromic portions in which the visible appearance changes afterlight exposure;

thermochromic portions in which the visible appearance changes as thenote is warmed;

solvatochromic effects in which the visible appearance changes as aresult of contact with solvent,

infrared revealable ink combinations, revealable when different viewingfilters are used on the real document.

In all of these cases the appearance of the image can change for onereason or another. The design system may hold data for the changedappearance, for example before and after exposure colour of aphotochromic material. In the case of materials which gradually changeappearance the data can be modified by colour mixing computationalroutines or the like.

As well as for embossed images where the lines are discernable such asin latent images the embossed pattern may be enhanced by a shading orshadowing routine to enhance the visual perception of the effect.

Certain features will possess angular variability, that is theirappearance will change depending on the angle of viewing. The system maybe provided with a three dimensional instruction set which would alloworthogonal projections to be viewed from an oblique viewing angle. Suchroutines may use stored or calculated data to give a three dimensionaloptically variable effect. Such computations are however complex and thecapability is unlikely to be used for the majority of designs.

By use of such three dimensional methods it is also possible to arrangeto view intaglio printing as relief effect whether on the whole note orsimply the intaglio printed layer.

The system may incorporate video effects which are used solely for themonitor display, to indicate reflective metal or diffraction of whitelight. If necessary these video effects may be kinetic in nature. Sucheffects may find use if the image information is to be displayed on adistantly connected monitor or, if the video display information istransmitted to a remote location.

The system may also take into account documents to which images are tobe attached such as identity cards by providing means for including aspecimen photograph, signature or fingerprint.

Many security documents are individually numbered so that every one fromthe security printed batch is rendered unique. The proofing system mayaccommodate this by showing representative numbers although anyspecified serial number can be printed, including using serial numbersin which adjacent number elements are in different font styles.

By reverse image manipulation the images on the printing plates may alsobe proofed.

FIG. 3 illustrates this high resolution colour monitor 5 connected toprocessor 1 which runs the system software. A keyboard 21 and mouse 22forming two of the input devices 2, are linked to the microprocessor 1.

In FIG. 4 the screen display 23 includes a window 24 for displaying thedocument being designed. Part 25 of the display 23 is reserved forsystem commands actuated by use of pointers under mouse control whichallow the designer to choose a colour, to initiate a procedure such asreducing the size of a feature, and for general system functionsincluding loading, saving and printing operations, usually via othermenus.

An area 26 displays the linear colour arrangement for images and theircombination.

The document 27 under design is confined within dimensions set in thelayout layer and denoted by a perimeter 28. The layout layer is also inthis instance causing the display of orthogonal centering lines 29.

There are three images displayed in the partially completed exampledocument 27 shown. The first comprises the background largely textmarkings 30. This is to be printed by lithography employing rainbowprinting layer L1. The initial ink colours and any limits of the rainbowbands are set by the designer at the bottom on the screen in the lowestof four horizontal colour defining zones shown at 26. The colour formarkings 30 is given by the colour defined in linear array 31. Here thedesigner has chosen a first ink colour and assigned that to the block100. A second ink colour is assigned to block 101 as shown and wherethey merge the system computes and represents the range of continuouslyblended colours in block 102. These continuously blended colours arestepwise approximated for display. Thus the image of the backgroundareas 30 is coloured according to that set in linear array 31. If theimage is moved laterally as the colour frame of reference remains fixed,individual image elements may take on new colours depending on whetherthey are moving into different colour areas.

The second lithographic image to be printed as layer L2 in the documentis that of the "continents". This is essentially a solid image and whereit does not overlie the printing of layer L1, the continent image adoptsthe colours defined in linear array 32 where a third ink colour isdefined to be within block 103 and a fourth ink colour in block 104 withtheir multilevel blend of colours assigned within block 105.

Where the solid continent image L2 overprints the text of L1 as shown bythe text lying within the continent areas, the colours resulting fromthe overprinting would take the colours of the weighted average of L1and L2 computed by the processor 1 and shown in linear array 33. Forexample L1 may be allowed to give a twenty percent colour contribution.This array contains derived colour for example with the colour for overprinting of images falling within blocks 100 and 103 resulting in theirbeing given the colour assigned to block 106. By analogy colours ofblocks 108 and 110 are similarly derived. Blocks 107 and 109 showrespective averages of blends 102 and colour 103, and blends 105 withcolour 101.

These X direction variations in colour are stored in the Litho ColourData Table 13 (FIG. 2). One array of this Table 13 will be provided foreach of the arrays 31, 32 and 33. If the selected layers of a displayedpixel has image content in one or both of the layers L1, L2 (asdetermined from the store 3) and does not contain an overprinting layer,then a value M (at the corresponding X location) will be selected fromthe array in the Table 13 corresponding to L1, L2 or L1+L2.

The document on display also includes an intaglio printing of a vectororigin head design 34. Converted to pixel image form for display. Thedensity of the intaglio ink such that if it prints on top of theunderlying lithographic layers its colour which will be the fifth inkcolour shown in array 35 and assigned totally to block 111, will obscurecompletely the underlying colours. In this instance the lithographicprinted area would marginally overlap (not shown) into the edges of theelliptical perimeter of the vector origin design to allow for slightvariations in registration during a printing run.

If the lithographic images were continuous under the vector design as itis now shown the intaglio image could be moved over the document and ineach new position it would take the colour defined for intaglio ink inthat location, the underlying images being suppressed.

In this example the variety of colours displayed is derived from onlyfive standard ink colours. This allows an unprecedented level ofaccuracy in colour image reproduction of security document to beattained.

The colour assignment for a proofing print will usually be made byresorting to the originally defined (8 bit) colours corresponding to thepure inks and the colours defined by the layers L1 to L4. intoequivalent YMCK values. The large number of YMCK values may have to belimited by a YMCK proofing colour approximating procedure so that thepixel colours fall within the range of proofing colour values permitted.These values are then used to control the yellow, magenta, cyan andblack inks in the ink jet printer 6. For example the individual Y, M, Cor K values may fall within thirty two levels.

The system may also allow a screen dump so that a proofing print of themonitor screen may be obtained for example by converting the displaycolour data directly into YMCK values, but generally this will not bedone if the proofing printer can print many more than 256 monitordisplay colours, thereby allowing a closer approximation to continuoustone colour.

The invention has been described with particular reference to banknotesbut it may be used in the design and manufacture of a wide range ofsecurity printed documents, such as travellers cheques, bank cheques,bonds, certificates including certificates of deposit, identity cards,permits, licences, ownership document forms, financial transactioninstruments such as plastic charge cards, credit cards, cash withdrawalcards and cheque guarantee cards, telephone or other service entitlementcards, vouchers, money orders, certificates of credit, access controlcards, passports, visas, lottery tickets, entrance tickets, traveltickets such as airline tickets including multiple leaf booklets, stampsincluding fiscal stamps and brand protection labels.

In addition to individual colour proof sheets being produced by thesystem, it is possible to place on one sheet a number of proofingimages. For example proofing images illustrating individual layers andtheir sequential composites as manufacture of the bank note progresses,may be shown. The ability, rapidly to prepare such progressive proofs,is a further useful feature of the system. Similarly it is possible toprepare a series of colour variations for a particular feature in onelayer so that the best aesthetic qualities can be achieved for the finalproduct.

A particularly useful aspect is the ability to offer not only truecolour proofing images but also to provide alternative colours. Analternative colour is one which may exist under certain circumstances,such as in the case of photochromic inks. Alternatively it may be usedto reveal the position of machine readable or discrete features, such asthe printing of magnetic inks, metameric inks or invisible butultraviolet (or infra red) illuminable luminescent inks.

The proofs may be presented at normal size or enlarged or reduced.Metallic effects and the like which are not readily achieved by currentproofing inks may be simulated by use of a colour, eg grey to representsilver tones. Alternatively the proofing image may be placed in registeron proofing paper which has already been given or subsequently given aspecial ink effect, such as metallic printed proofing paper stock orholographically labelled stock or threaded stock.

In summary proofing prints may represent on a single sheet.

1. one or both sides of a completed document with optionally the markingof hidden or secondary features:

2. individual images including the base paper images including hidden orsecondary image positions;

3. the sequence of individual layers and their composites following thestages of manufacture of the document and the final document (that is aseries of progressive proofs) with optionally the separate marking ofhidden or secondary features; or

4. a portion of an imaged web at any stage of production showingmultiples of the same document stock or combinations of documents.

A further important aspect of the invention that the processor can storea set of "rules" which constrain in a predetermined manner the way inwhich images may be placed in a design as well as the results ofoverlaying described above. For example, the rules may limit placementof a serial number to certain regions.

The computer program for controlling the apparatus in accordance withthe invention may be run on a Sun Microsystems computer systemcomprising Sun 4 hardware and the Sun operating system, version 4.0.

Apart from the facilities already described, routines for which may bewritten by those skilled in the computer programming art, the system mayalso allow the presentation of ruling marks and measurement marks on thescreen or the proofing printout which marks need not necessarily be heldwithin the image data stack.

We claim:
 1. Apparatus for handling digital representations of documentswhich are to be provided with one or more images, the apparatuscomprising a first store for storing digital data defining the imagecontent of each pixel of each image; color data generating means forgenerating data defining the color of each pixel of each image orcombination of images as defined by a user, separate from the imagecontent data; and processing means for selectively combining the imagecontent data and color data to enable selected images to be viewedseparately or in combination, wherein the color data generating meanscomprises a second store in the for of a look-up table which has anaddress corresponding to each possible combination of images within apixel and contains at each address color data defining the resultantpixel color.
 2. Apparatus according to claim 1, wherein the first storecontains at least one binary digit for each pixel of each image. 3.Apparatus for handling digital representations of documents which are tobe provided with one or more images, the apparatus comprising a firststore for storing digital data defining the image content of each pixelof each image; color data generating means for generating data definingthe color of each pixel of each image or combination of images asdefined by a user, separate from the image content data; and processingmeans for selectively combining the image content data and color data toenable selected images to be viewed separately or in combination,wherein the color data generating means generates color data for atleast one image which varies with position in one direction across thedocument.
 4. Apparatus according to claim 3 wherein the color datagenerating means is adapted to generate color data for at least oneimage which varies with position in orthogonal directions across thedocument.
 5. Apparatus according to claim 1, wherein the processingmeans includes means for adapting the combined image content and colordata for use with a display device.
 6. Apparatus for handling digitalrepresentations of documents which are to be provided with one or moreimages, the apparatus comprising a first store for storing digital datadefining the image content of each pixel of each image; color datagenerating means for generating data defining the color of each pixel ofeach image or combination of images as defined by a user, separate fromthe image content data; and processing means for selectively combiningthe image content data and color data to enable selected images to beviewed separately or in combination, wherein the processing meansincludes means for adapting the combined image content and color datafor use with a display device and the adapting means comprises a look-uptable for storing control values suitable for use with the displaydevice, the look-up table having an address corresponding to everypossible combination of image content and color data value.
 7. Apparatusfor handling digital representations of documents which are to beprovided with one or more images, the apparatus comprising a first storefor storing digital data defining the image content of each pixel ofeach image; color data generating means for generating data defining thecolor of each pixel of each image or combination of images as defined byuser, separate from the image content data; and processing means forselectively combining the image content data and color data to enableselected images to be viewed separately or in combination, wherein theprocessing means includes means for adapting the combined image contentand color data for use with a display device and the processing meansgenerates from the image content and color data proofing control signalsdefining the corresponding image or combined images in terms of proofinginks or dyes.
 8. Apparatus according to claim 7, wherein the proofinginks or dyes comprise two or more of cyan, magenta, yellow, and black.9. Apparatus for handling digital representations of documents which areto be provided with one or more images, the apparatus comprising a firststore for storing digital data defining the image content of each pixelof each image; color data generating means for generating data definingthe color of each pixel of each image or combination of images asdefined by user, separate from the image content data; processing meansfor selectively combining the image content data and color data toenable selected images to be viewed separately or in combination; and aframe store for connection to a monitor, the processing means storingcombined image content and color data for each pixel to be displayed inthe frame store.
 10. Apparatus for handling digital representations ofdocuments which are to be provided with one or more images, theapparatus comprising a first store for storing digital data defining theimage content of each pixel of each image; color data generating meansfor generating data defining the color of each pixel of each image orcombination of images as defined by a user, separate from the imagecontent data; and processing means for selectively combining the imagecontent data and color data to enable selected images to be viewedseparately or in combination, wherein at least one imaging operationcomprises a printing operation and the processing means generates datawhich defines the color content of pixels of the corresponding image orcombined images in terms of ink or dye densities.
 11. Apparatusaccording to claim 10, wherein the printing operation comprisesdepositing a quantity of ink in each area corresponding to a pixel, thequantity of ink deposited corresponding to the data value for the pixel.12. Apparatus according to claim 10, wherein the ink or dye densitiesare defined on a scale having at least three values.
 13. Apparatusaccording to claim 12, wherein the scale has at least twenty values. 14.Apparatus according to claim 10, wherein each printing operation isselected from wet lithography, dry lithography, intaglio and letterpressprinting.
 15. Apparatus according to claim 14, wherein said printingoperation is one of wet and dry lithography and wherein said processingmeans is adapted to generate at least one image with a rainbow effect.16. Apparatus according to claim 14, wherein said printing operation isintaglio and wherein said color data defines schablones.
 17. Apparatusaccording to any of claim 9, wherein the processing means generatescontrol data for controlling the manufacture of a printing plate. 18.Apparatus according to claim 1, wherein each document is to be providedwith at least one image.
 19. Apparatus according to claim 1, wherein theprocessing means registers the images with respect to one another. 20.Apparatus for handling digital representations of documents according toclaim 1, wherein the documents are security documents.
 21. Apparatusaccording to claim 1, wherein the color data generating means generatescolor data for at least one image which varies with position in onedirection across the document.
 22. Apparatus according to claim 21,wherein the color data generating means is adapted to generate colordata for at least one image which varies with position in orthogonaldirections across the document.
 23. Apparatus according to claim 1,wherein the processing means includes means for adapting the combinedimage content and color data for use with a display device. 24.Apparatus according to claim 23, wherein the adapting means comprises alook-up table for storing control values suitable for use with thedisplay device, the look-up table having an address corresponding toevery possible combination of image content and color data value. 25.Apparatus according to claim 23, wherein the processing means is adaptedto generate from the image content and color data proofing controlsignals defining the corresponding image or combined images in terms ofproofing inks or dyes.
 26. Apparatus according to claim 25, wherein theproofing inks or dyes comprise two or more of cyan, magenta, yellow, andblack.
 27. Apparatus according to claim 1, further comprising a framestore for connection to a monitor, the processing means storing combinedimage content and color data for each pixel to be displayed in the framestore.
 28. Apparatus according to claim 1, wherein at least one imagingoperation comprises a printing operation, wherein the processing meansgenerates data which defines the color content of pixels of thecorresponding image or combined images in terms of ink or dye densities.29. Apparatus according to claim 28, wherein the printing operationcomprises depositing a quantity of ink in each area corresponding to apixel, the quantity of ink deposited corresponding to the data value forthe pixel.
 30. Apparatus according to claim 28, wherein the ink or dyedensities are defined on a scale having at least three values. 31.Apparatus according to claim 30, wherein the scale has at least twentyvalues.
 32. Apparatus according to claim 28, wherein each printingoperation is selected from wet lithography, dry lithography, intaglioand letterpress printing.
 33. Apparatus according to claim 32, whereinsaid printing operation is one of wet and dry lithography and whereinsaid processing means is adapted to generate at least one image with arainbow effect.
 34. Apparatus according to claim 32, wherein saidprinting operation is intaglio and wherein said color data definesschablones.
 35. Apparatus according to claim 27, wherein the processingmeans is adapted to generate control data for controlling themanufacture of a printing plate.
 36. A security document proofmanufactured by an apparatus, comprising:a first store for storingdigital data defining the image content of each pixel of each image, thesecurity document being a composite having at least one image; colordata generating means for generating data defining the color of eachpixel of one of a single image and a combination of images as defined bya user, separate from the image content data; and processing means forselectively combining the image content data and the color data toenable selected images to be viewed as one of a separate image and acombination of images, said processing means generating from the imagecontent data and the color data proofing control signals defining thecorresponding one of the separate image and the combination of images interms of proofing inks and proofing dyes, wherein the processing meansincludes means for adapting the combined image content data and colordata for use with a display device.
 37. A security document proofaccording to claim 36, wherein the color generating means generatescolor data for at least one image which varies with position inorthogonal directions across the document.
 38. A security document proofsheet having more than one security document proof thereon, the proofsillustrating the results of different combinations of imagingoperations, wherein the security document proof sheet is produced by anapparatus for handling representations of documents which are to beprovided with one or more images, said apparatus comprising:a firststore for storing digital data defining the image content of each pixelof each image; color data generating means for generating data definingthe color of each pixel of each image or combination of images asdefined by a user separate from the image content data; and processingmeans for selectively combining the image content data and color data toenable selected images to be viewed separately or in combination.
 39. Asheet according to claim 38, wherein the proofs illustrate the resultsof successive imaging operations, there being one proof for each imagingoperation.